The present invention concerns a procedure to measure at least a component of a force tenser applied to a part, having an axis of symmetry, the force tenser defined as follows in a trigonometric coordinate system:
Fx Fy Fz
Mx My Mz,
this procedure requiring fixing at least two identical deformation measurement sensors on said part, each being directly mounted between two distinct anchoring points using fastening members designed to transmit to the sensors deformations of said part, said sensors being designed to deliver electric signals as a function of a change in distance between said respective anchoring points caused by said deformations, proceeding to electrically condition said signals from said sensors, this electrically conditioning being specifically associated with said sensors, and combining these signals to deduct a value of said component of said force tenser.
It also concerns a measurement device to realize the above procedure, each sensor having two contacts spaced apart by a distance equivalent to that defined between the anchoring points and a pretensioned blade mounted at its extremities in said contacts and having at least one sensing element.
This procedure and measurement device are particularly suited for fixed shafts subject to a reaction torque to a breaking torque or a motor torque. They can however be used for rotating shafts, the measurement device being, in this case, associated with a contact or contactless electric transmission means. The shaft to control is not necessarily cylindrical and its cross section not necessarily circular.
In order to improve the maintenance of mechanical equipment in general, it is useful to monitor and control parts subjected to forces, notably by fixing sensors, which permit static or dynamic behavior information to be obtained, on the parts. This can be done, for example, using a measurement of the deformation.
In the case of ground or air vehicle braking, knowledge of wheel shaft deformations permit precise indications of the exerted braking torque to be given, independently from wheel slide or deformation for example. Knowing this braking torque notably permits improvements in braking performance control.
In many applications, a part subject to a torque is also deformed due to bending loads. Very often, these bending loads interfere with torque measurement and it is necessary to eliminate them. More generally, one can wish, during measurements, to keep certain force tensor components as defined above and eliminate others.
Known mechanical deformation sensors vary diversely in nature and performance. Many different technologies can be used to measure torque. However, few permit torque measurement with a simple and inexpensive device that can be easily fixed to a part to monitor without requiring additional instrumentation assemblies.
Indeed, instrumentation of the part to be monitored can be done by directly fixing strain gauges to the part, choosing the number of strain gauges, their locations and directions, as described in numerous mechanical deformation instrumentation manuals. These installations, are very costly, difficult and impossible to achieve outside specialized shops or laboratories having highly qualified personnel. Attempts are often made to avoid these difficulties by equipping sensor proof bodies with deformation gauges, which are then inserted between two elements of the piece to monitor. In this case for example, with test cell torque meters which are inserted between the motor and driven shafts in a power transmission installation.
This is also the case for many devices such as those described in the publications DE-A-3 406 059 and 3 405 168 or EP-A-0 410 133. These devices remain costly and specific and require the test part to be modified and as such are sometimes impossible to install, since the modifications required are too great. A number of these devices must be cleaned of the interferences (bench-mounted torque meters).
On the other hand, the publication U.S. Pat. No. 3,780,817 describes a sensor limited to bending load measurement and comprised of a pretensioned flexible blade mounted between two fixed contacts, the blade having strain gauges thereon. Its major inconvenience resides in the fact that it is not adjustable, as the fixation points are permanently fixed. Therefore it is not possible to preload the sensor before or during use, in order to achieve a predetermined operation point. In the publication U.S. Pat. No. 5,585,572, the torsion sensor disclosed is very complex and cumbersome. It requires a very costly implementation and cannot be used on shafts to which there is limited access, which cannot be disassembled or which are not cylindrical.
Other devices use a variation in capacitance of a deformable air gap, or an optical measurement system. They use, in general, the rotation of adjacent sections and measure a variation in length that is representative of the angle. In the assembly described in the publication U.S. Pat. No. 4,941,363 for example, the two plates of the capacitor are mounted on collars clamped to the shaft, the reliability of the clamped collars is very difficult to ensure. Additionally, clamping the collars is impossible if the shaft is not cylindrical. It is very difficult to eliminate all interference constraints. The majority of these devices are, additionally, sensitive to external influences.
Other types of torsion sensors also exist, such as that described in the publication U.S. Pat. No. 5,831,180 which uses magnetostriction. The sensor is limited to torque measurement on a vehicle steering bar, where torsion forces are low and bending moment interferences for example, are almost non-existent. This sensor has a complex shape adapted to the geometry of the bar, which does not permit industrial reproducibility at reduced costs. Additionally, its measurement principle is very sensitive to temperature changes, and its engagement on the shaft does not permit any adjustability. The measurement principle based on using two sensors which are symmetrical with respect to a plane passing through the axis of the bar does not automatically permit interference effects to be eliminated, as the two sensors are in opposition. This sensor and its measurement procedure are therefore not at all suitable for the particular application of the present invention.
The present invention has for an object to overcome these inconveniences by providing a torque measurement procedure as well as a device for the implementation of this procedure, the device being easily industriallisable and inexpensively reproducible, being very precise, of minimal bulk, capable of being adapted for parts having various shapes, easy to install and disassemble without having to disassemble any surrounding parts, resistant to temperature variations, adjustable once mounted on the and capable of using specific sensors or those available commercially, the sensors being capable of measuring a length deformation without being subjected to significant forces at their mounting points.
This object is achieved by the procedure as defined in the preamble, characterized by the two distinct anchoring points of each sensor on a surface of the part, such that the two lines passing respectively through the two distinct anchoring points of each sensor, reported on the surface of the part are substantially parallel and form a line parallel to the axis of symmetry of the part going through one of the said anchoring points, forming an angle xcex1 relative to the axis of symmetry of the part. The angle xcex1 between 0 and 90 degrees. Preferably angle xcex1 is equal to 45 degrees.
In accordance with the various arrangements of said sensors, they can be disposed diametrically opposite on said part.
The object can also be achieved by the device as defined in the preamble and characterized in that at least one contact comprises regulation means arranged to adjust the pretension on said blade.
This device is advantageous when applied to large parts of limited maneuverability, being part of a complex system that cannot be modified or which requires significant instrumentation.
Each contact preferably comprises a safety jaw having a slot for receiving the corresponding extremity of said pretensioned blade being,fixed in said contact by a location. screw. Regulation means can comprise a control screw engaged with at least one safety jaw for adjusting its position in the direction of said pretension blade.
Preferably, the torque instruments comprise anchoring contacts, each anchoring contact comprising a top face, fixed jointly to a sensor contact and a bottom face having a shape adapted to conform to said part and being fixed jointly thereto.
The contact can be fixed to the anchoring contact corresponding to the removable fixation medium and the anchoring contact can be fixed to said part using a fixation means chosen from along gluing, welding, screwing, and molding.
Preferably, the anchoring contacts are composed of-a rigid non-shrinking material, such as a metal or metal alloy.
According to the characteristics of said sensors on the part, the calculator is chosen from among an adder and a subtractor, either digital or analog.
The principle advantage of the method according to the present invention are that it is capable of providing precise and reliable information relating to the torque exerted on a part, using a minimum number of sensors, notably two, while automatically eliminating interfering forces even if they are of significant size, automatically compensating for any eventual dilations due to temperature variations, and using a simple conditioning circuitry arranged near the sensors and having a simple analog or digital processor.